Equiatomic and near-equiatomic nickel-titanium alloys possess both “shape memory” and “superelastic” properties. More specifically, these alloys, which are commonly referred to as “Nitinol” alloys, are known to undergo a martensitic transformation from a parent phase (commonly referred to as the austenite phase) to at least one martensite phase on cooling to a temperature below the martensite start temperature (“Ms”) of the alloy. This transformation is complete on cooling to the martensite finish temperature (“Mf”) of the alloy. Further, the transformation is reversible when the material is heated to a temperature above its austenite finish temperature (“Af”).
This reversible martensitic transformation gives rise to the shape memory properties of the alloys. For example, a nickel-titanium shape-memory alloy can be formed into a first shape while in the austenite phase (i.e., at a temperature above the Af of the alloy), subsequently cooled to a temperature below the Mf, and deformed into a second shape. As long as the material remains below the austenite start temperature (“As”) of the alloy (i.e., the temperature at which the transition to austenite begins), the alloy will retain the second shape. However, if the shape-memory alloy is heated to a temperature above the Af, the alloy will revert back to the first shape if not physically constrained, or when constrained can exert a stress upon another article. Recoverable strains of up to 8% are generally achievable with nickel-titanium alloys due to the reversible austenite-to-martensite thermally-induced transition, and hence the term “shape-memory.”
The transformation between the austenite and martensite phases also gives rise to the “pseudoelastic” or “superelastic” properties of shape-memory nickel-titanium alloys. When a shape-memory nickel-titanium alloy is strained at a temperature above the Af of the alloy but below the so-called martensite deformation temperature (“Md”), the alloy can undergo a stress-induced transformation from the austenite phase to the martensite phase. The Md is therefore defined as the temperature above which martensite cannot be stress-induced. When a stress is applied to a nickel-titanium alloy at a temperature between Af and Md, after a small elastic deformation, the alloy yields to the applied stress through a transformation from austenite to martensite. This transformation, combined with the ability of the martensite phase to deform under the applied stress by movement of twinned boundaries without the generation of dislocations, permits a nickel-titanium alloy to absorb a large amount of strain energy by elastic deformation without plastically (i.e., permanently) deforming. When the strain is removed, the alloy is able to revert back to its unstrained condition, and hence the term “pseudoelastic.” Recoverable strains of up to 8% are generally achievable with nickel-titanium alloys due to the reversible austenite-to-martensite stress-induced transition, and hence the term “superelastic.” Thus, superelastic nickel-titanium alloys macroscopically appear to be very elastic relative to other alloys. The terms “pseudoelastic” and “superelastic” are synonymous when used in connection with nickel-titanium alloys, and the term “superelastic” is used in this specification.
The ability to make commercial use of the unique properties of shape-memory and superelastic nickel-titanium alloys is dependent in part upon the temperatures at which these transformations occur, i.e., the As, Af, Ms, Mf, and Md of the alloy. For example, in applications such as vascular stents, vascular filters, and other medical devices, it is generally important that nickel-titanium alloys exhibit superelastic properties within the range of in vivo temperatures, i.e., Af≦˜37° C.≦Md. It has been observed that the transformation temperatures of nickel-titanium alloys are highly dependent on composition. For example, it has been observed that the transformation temperatures of nickel-titanium alloys can change more than 100 K for a 1 atomic percent change in composition of the alloys.
In addition, various applications of nickel-titanium alloys, such as, for example, actuators and implantable stents and other medical devices, may be considered to be fatigue critical. Fatigue refers to the progressive and localized structural damage that occurs when a material is subjected to cyclic loading. The repetitive loading and unloading causes the formation of microscopic cracks that may increase in size as a material is further subjected to cyclic loading at stress levels well below the material's yield strength, or elastic limit. Fatigue cracks may eventually reach a critical size, resulting in the sudden failure of a material subjected to cyclic loading. It has been observed that fatigue cracks tend to initiate at non-metallic inclusions and other second phases in nickel-titanium alloys. Accordingly, various applications of nickel-titanium alloys, such as, for example, actuators, implantable stents, and other fatigue critical devices, may be considered to be inclusion and second phase critical.